Nutrient availability increases photosynthetic capacity without altering the cost of resource use for photosynthesis

Eco-evolutionary optimality (EEO) theory predicts that plants maximize resource investment in photosynthetic capacity at the lowest costs of acquiring and using such resources. However, current EEO-based models predict photosynthetic capacity based on climate alone, and omit costs for resource acquisition. To explore the link between leaf-level optimality and plant-level nitrogen acquisition costs across different soil environments, we grew two commonly co-occurring species in a greenhouse under three nutrient fertilization levels in sand and two natural soils with matching nutrient availability to the fertilization levels in sand. At the end of the experiment, we measured the maximum rate of Rubisco carboxylation (<i>V</i> _cmax), δ¹³C-derived leaf-to-air CO₂ partial pressure ratio (<i>c_i</i> /<i>c_a</i> ), and structural carbon costs for nitrogen acquisition. Increasing nutrient availability increased <i>V</i> _cmax (<i>P</i> < .001) and decreased carbon costs for nitrogen acquisition (<i>P</i> < .001), similarly in sand and natural soils (<i>P</i> > .1 for both). Yet, the leaf <i>c_i</i> /<i>c_a</i> remained unchanged across treatments in sand (<i>P</i> = .426) and natural soils (<i>P</i> = .499), consistent with the current EEO-models assumption of climate-dependent optimality. These findings support the general principle that nutrient scarcity increases acquisition costs, while also highlighting a gap in current model formulations that neglect nutrient effects on photosynthetic acclimation.